Hammer having a two part body

ABSTRACT

A tool assembly such as a hammer assembly, includes a housing defining a chamber and a reciprocating piston arranged in the chamber for cyclical movement in a work stroke and a return stroke. The housing includes a lower housing member, an upper housing member and an engaging structure which permits the engagement of the upper housing member with the lower housing member by relative rotation of the upper and lower housing members, such that each of the lower and upper housing members defines a portion of the chamber. The upper housing member is a precision component which contains the hydraulic components or other drive components necessary to drive the piston, while the lower housing member can be manufactured separately since it does not need to be manufactured to the high tolerances required by the upper housing member. The resulting tool assembly may be more compact than a conventional tie rod tool assembly.

This application is a continuation of U.S. patent application Ser. No.11/452,324 filed Jun. 14, 2006.

TECHNICAL FIELD

The present disclosure relates generally to a tool assembly, and morespecifically to a hammer assembly having a two part body comprising anupper housing member and a lower housing member.

BACKGROUND

Hydraulic hammers are used on work sites to break up large hard objectsbefore such objects can be moved away. Hydraulic hammers may be mountedto back hoes or excavators, or may be hand-held. Typically, the hammerassembly is powered by either a hydraulic or pneumatic pressure source.During a work or power stroke, high fluid pressure is applied to a firstshoulder of a piston, thereby driving the piston in a forward direction.The piston then strikes a work tool, which is driven in the forwarddirection thereby causing a work tip of the tool to strike the rock,concrete, asphalt or other hard object to be broken up. During a returnstroke, fluid pressure is applied to a second shoulder of the piston inorder to return the piston to its original position.

The work tool, also known as a tool bit, is retained within a sleeve,commonly referred to as a front head. When the tool is driven forward,the kinetic energy of the tool is transferred to the object to be brokenup. However, if full contact with the object to be broken up does notoccur, a proportion of the energy is transferred to the front head. Ifno contact takes place with the object to be broken up, then all of theenergy may be transferred to the front head. Consequently, the fronthead is subject to cyclical loading which tends to pull it away from thecylinder of the hammer assembly. Conventionally, external tie rods areused to tie the front head to the cylinder. The tie rods increase theoverall diameter of the hammer assembly and add to the complexity ofmanufacture. Tie rods typically have to be replaced at regular serviceintervals, since they are subject to fatigue failure and failure of atie rod can cause irreparable damage to the entire hammer assembly.

SUMMARY OF THE INVENTION

A method may comprise attaching an upper housing member to a lowerhousing member to form a housing, the housing including a chamberextending between the upper housing member and the lower housing member,the upper housing member having a first pressure source of pressurizedhydraulic fluid or pressurized air, positioning a piston inside thechamber for reciprocal movement therein, the piston including at least afirst surface which communicates with the first pressure source to movethe piston in a first direction, positioning a reciprocating work toolat least partially inside of the chamber, an upper part of the work toolbeing struck by the piston as the piston moves in the first directioninside the chamber to transfer force from the piston to a tip of thework tool, the tip being formed opposite the upper part and residingoutside of the chamber, the upper part of the work tool being housedinside the lower housing member, removably attaching a tool retentionmember to the lower housing member, wherein when the tool retentionmember is attached to the lower housing member the tool retention memberblocks removal of the work tool from the chamber and detachment of thework tool from the lower housing member, and when the tool retentionmember is removed from the lower housing member, the work tool may beremoved from the chamber and detached from the lower housing member forreplacement, and attaching the upper housing member to the lower housingmember further comprises engaging a socket portion formed on one of theupper housing member or the lower housing member with a plug portionformed on the other of the upper housing member or the lower housingmember, by inserting a plurality of outwardly projecting lugs formed onthe plug portion into the socket and between a plurality of inwardlyprojecting projections formed on the socket portion, then rotating thelugs relative to the projections to engage the lugs beneath theprojections and prevent removal of the plug portion from the socketportion.

A hammer may comprise a housing including an upper housing member and alower housing member, the housing defining a chamber, the upper housingmember having a first pressure source of pressurized hydraulic fluid orpressurized air, a piston reciprocating inside the chamber, the pistonincluding at least a first surface which communicates with the firstpressure source to move the piston in a first direction, a reciprocatingwork tool residing at least partially inside of the chamber, an upperpart of the work tool being struck by the piston as the piston moves inthe first direction inside the chamber to transfer force from the pistonto a tip of the work tool, the tip being formed opposite the upper partand residing outside of the chamber, the upper part of the work toolbeing housed inside the lower housing member, a tool retention memberremovably attached to the lower housing member, wherein when the toolretention member is attached to the lower housing member the work toolcannot be removed from the chamber and when the tool retention member isremoved from the lower housing member, the work tool may be removed fromthe chamber for replacement, and a socket portion formed on one of theupper housing member or the lower housing member, a plug portion formedon the other of the upper housing member or the lower housing member,the plug portion positioned in the socket portion, a plurality ofoutwardly projecting lugs formed on the plug portion and a plurality ofinwardly projecting projections formed on the socket portion, the lugsbeing passable between the projections when the upper housing member isattached to the lower housing portion by inserting the plug portion intothe socket portion, then rotatable relative to the projections to engagethe lugs beneath the projections and prevent removal of the plug portionfrom the socket portion.

A hammer may comprise a housing including an upper housing member and alower housing member, the housing defining a chamber, the upper housingmember having a first pressure source of pressurized hydraulic fluid orpressurized air, a piston reciprocating inside the chamber, the pistonincluding at least a first surface which communicates with the firstpressure source to move the piston in a first direction, a reciprocatingwork tool residing at least partially inside of the chamber, an upperpart of the work tool being struck by the piston as the piston moves inthe first direction inside the chamber to transfer force from the pistonto a tip of the work tool, the tip being formed opposite the upper partand residing outside of the chamber, the upper part of the work toolbeing housed inside the lower housing member, a tool retention memberremovably attached to the lower housing member, wherein when the toolretention member is attached to the lower housing member the work toolcannot be removed from the chamber and when the tool retention member isremoved from the lower housing member, the work tool may be removed fromthe chamber for replacement, and engaging structure associated with theupper housing member and the lower housing member and configured topermit the engagement and disengagement of the upper housing member withthe lower housing member by relative rotation of the upper housingmember and the lower housing member, the engaging structure includingmutually engaging mating surfaces on the upper housing member and lowerhousing member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a hammer assembly according to onedisclosed embodiment;

FIG. 2 is an exploded view of the hammer assembly of FIG. 1;

FIG. 3 shows a longitudinal section of a hammer assembly according toanother disclosed embodiment;

FIG. 4 is a cross-sectional view on line IV-IV through the hammerassembly of FIG. 3;

FIG. 5 shows a longitudinal section of another hammer assembly accordingto yet another disclosed embodiment; and

FIG. 6 is a enlarged view of part of the longitudinal section of FIG. 5.

DETAILED DESCRIPTION

With reference to FIGS. 1 and 2 there is shown a tool assembly 10,specifically a hydraulic hammer assembly, which may be attached to abackhoe or excavator (not shown). The tool assembly 10 includes ahousing 11, a chamber 16 defined in the housing 11, a piston 18 and awork tool 20. The housing 11 is a two part housing including an upperhousing member 12 and a lower housing member 14, which define an upperand lower chamber respectively, which together make up the chamber 16.The upper housing member 12 may be referred to as a cylinder, while thelower housing member 14 may be referred to as a front head. The piston18 is operatively housed in the chamber 16 such that the piston 18 cantranslate in the general direction of arrows 22 and 24. In particular,during a work stroke, the piston 18 moves in the general direction ofarrow 22 so as to strike the work tool 20, the upper part of which ishoused in the chamber 16 of the front head 14. Conversely, during areturn stroke, the piston 18 moves in the general direction of arrow 24.

The piston 18 includes a work portion 26 and a load portion 28.Pressurized fluid is advanced by a first hydraulic pressure source 30against the work portion 26 to serve as a drive structure and urge thepiston 18 towards the work tool 20 during the work stroke. Similarly,pressurized fluid is advanced by a second hydraulic pressure source 32against the load portion 28 to serve as a return structure to return thepiston 18 during the return stroke.

The hydraulic circuit is not described further, since it will beapparent to the skilled person that any suitable hydraulic arrangementmay be used to provide pressurised fluid to the piston 18, such as thearrangement described in U.S. Pat. No. 5,944,120.

Near the end of the work stroke, the piston 18 strikes the work tool 20.The work tool 20 includes a tip 46. A tool retention member 80 retainsthe work tool 20 and limits the movement of the work tool 20 in thegeneral direction of arrow 22. Removing the tool retention member 80allows a variety of work tools 20 with different configurations of thetip 46 to be attached to the tool assembly 10. As the piston 18 strikesthe work tool 20, the force of the piston 18 is transmitted through thework tool 20 to the tip 46 in the general direction of arrow 22.Moreover, this force is applied to a hard object such as rock, concrete,or asphalt in order to break up the hard object. The tool retentionmember does not form part of the present disclosure but is described inmore detail in co-pending U.S. patent application Ser. No. 11/452,381,filed Jun. 15, 2006.

FIGS. 3 and 4 show a hammer assembly according to another embodiment.This differs from that shown in FIGS. 1 and 2 only in the tool retentionmember which is in the form of a retaining pin 148 which engages with agroove 150 in the tool 20.

With reference to the embodiments of FIGS. 1 to 4 the upper and lowerhousing members 12, 14 are connected by an engaging structure 50. In theillustrated embodiment the engagement structure 50 includes four lugs 52provided equidistantly about the circumference of a plug portion 54 ofthe upper housing member 12. These are shaped such that as the plugportion 54 is inserted into a socket portion 56 of the lower housingmember 14 the lugs 52 can pass between four projections 58 formed on theinternal wall 60 of the socket portion 56. The upper housing member 12is then rotated by 45 degrees relative to the lower housing member 14 sothat the lugs 52 engage beneath the projections 58. It is to beunderstood that other forms of mutually engaging protrusions may beenvisaged, and the shape and number of lugs and protrusions can bevaried and are not limited to those shown in the figures. A stop (notillustrated) may be formed on the internal wall 60 of the socket portionsuch that upon rotation of the upper housing member 12 one of the lugs52 comes into contact with the stop to indicate that sufficient rotationhas taken place.

Prestressing structure in the form of four jacking screws 70 areprovided to urge the upwardly facing mating surfaces 72 of the lugs 52against the downwardly facing mating surfaces 74 of the projections 58.The screws 70 engage in threaded apertures 76 provided equidistantlyaround the perimeter of a circular flange 78 of the upper housing member12. The jacking screws 70 engage with an upwardly facing bearing surface79 formed on the upper end of the lower housing member 14.

Although embodiments have been described with reference to lugs andprojections, it is to be understood that other forms of mutuallyengaging protrusions may be provided. For example the upwardly facingmating surface on the upper housing member may be provided on a helicalprotrusion which forms a male thread and engages with the downwardlyfacing mating surface of a corresponding helical protrusion on the lowerhousing member, serving as a female thread. In another example theengaging structure may be a bayonet coupling in which two or more pinsare provided on the plug of the upper housing member, the pins engagingwith two or more L-shaped slots provided on the internal surface of thesocket.

Although an embodiment has been described in which the plug portion 54is formed on the upper housing member 12 and the socket portion 56 isformed on the lower housing member 14, the engaging structure may bereversed, so that the plug portion is formed on the lower housing member14 and the socket portion is formed on the upper housing member 12.

Although the prestressing structure described above comprises fourjacking screws 70, the number, type and position of the jacking screwsmay be varied. Moreover other prestressing structure may be used, suchas springs or other resilient elements which urge the upper and lowerhousing members apart. In the case of corresponding helical protrusionson the upper and lower housing members the prestressing structure may bethe elastic or plastic deformation of the threads under a tighteningtorque on the housing members.

FIGS. 5 and 6 show a hammer assembly according to another embodiment.This differs from that shown in FIGS. 1 to 4 in that tension screws areused instead of compression jacking screws to impart the prestress. Thetool assembly 210 includes a housing 211, a chamber 216 defined in thehousing 211, a piston 218 and a work tool 220. The housing 211 is a twopart housing including an upper housing member 212 and a lower housingmember 214, which define an upper and lower chamber respectively, whichtogether make up the chamber 216. The piston 218 is operatively housedin the chamber 216 such that the piston 218 can translate in the generaldirection of arrows 222 and 224. In particular, during a work stroke,the piston 218 moves in the general direction of arrow 222 so as tostrike the work tool 220. Conversely, during a return stroke, the piston218 moves in the general direction of arrow 224. The operation of thehydraulic circuit, piston 218, work tool 220 and tool retention member280 is as described above with reference to FIGS. 1 and 2, and is notdescribed further.

The upper and lower housing members 212, 214 are connected by anengaging structure 250. In the illustrated embodiment the engagementstructure 250 includes four lugs 252 provided equidistantly about thecircumference of a plug portion 254 of the upper housing member 212.These are shaped such that as the plug portion 254 is inserted into asocket portion 256 of the lower housing member 214 the lugs 252 can passbetween four projections 258 formed on the internal wall 260 of thesocket portion 256. The upper housing member 212 is then rotatedrelative to the lower housing member 214 so that the lugs 252 engagebeneath the projections 258. It is to be understood that other forms ofmutually engaging protrusions may be envisaged, and the shape and numberof lugs and protrusions can be varied and are not limited to those shownin the figures. A stop (not illustrated) may be formed on the internalwall 260 of the socket portion such that upon rotation of the upperhousing member 212 one of the lugs 252 comes into contact with the stopto indicate that sufficient rotation has taken place.

An annular bearing surface 300 is provided on the internal wall 260 ofthe socket portion 256. A piston sleeve 302 is provided around thepiston and bears at one end against the bearing surface 300. The otherend of the piston sleeve 302 bears against a valve sleeve 304 which isheld against an accumulator housing 306. The accumulator housing 306 andits associated accumulator cover 308 are secured to an upper bearingsurface 314 of the upper housing member 212 by tension screws 310. Thevalve sleeve 304 has a resilient deflector flange 312 which deflects toprovide a biasing compression force when a torque is applied to thetension screws 310, as described below. The tension screws may be neckeddown, and may form part of a Bellville type interface.

INDUSTRIAL APPLICABILITY

The operation of the embodiment of the tool assembly shown in FIGS. 1 to4 will be described. In use, the jacking screws 70 are tightened to apredetermined torque to prestress the engaging structure and establish aknown contact stress between the upwardly facing mating surfaces 72 ofthe lugs 52 and the downwardly facing mating surfaces 74 of theprojections 58.

In the embodiment shown in FIGS. 5 and 6, as the tension screws 310 aretightened the accumulator housing 306 contacts the resilient deflectorflange 312 before it contacts the upper bearing surface 314 of the upperhousing member 212. The flange 312 urges the valve sleeve 304 intocompressive contact with the piston sleeve 302, which in turn bearsagainst the annular bearing surface 300. As the tension in the screws310 increases the compressive force applied from the accumulator housing306 to the valve sleeve 304 increases, until the accumulator housing 306contacts the upper bearing surface 314 of the upper housing member 212.At this point the resilient deflector flange 312 is fully deflected, andthe lower housing member 214 is urged away from the upper housing member212 by the liner stack, comprising the valve sleeve 304 and pistonsleeve 302, thereby establishing a predetermined contact stress betweenthe upwardly facing mating surfaces 272 of the lugs 252 and thedownwardly facing mating surfaces 274 of the projections 258.

In at least some of the embodiments, under the cyclical action of thepiston 18, 218 and tool 20, 220, which act to pull the upper 12, 212 andlower 14, 214 housing members apart, the contact stress variescyclically, but remains in compression throughout the cycle, therebyreducing the risk of fatigue failure. In contrast the tie rods of theprior art hammer systems are subject to cyclical tensile stresses andhave a much greater risk of fatigue failure.

The upper housing member 12, 212 is a precision component which containsthe hydraulic components of the hammer system and requires substantialmachining, while the lower housing member 14, 214 is a lower precisioncomponent which requires minimal manufacturing. By manufacturing the twohousing members 12, 212, 14, 214 as separate components and joining themby the engaging structure 50, 250, the lower housing member can be mademore economically and does not have to be manufactured to the hightolerances required by the upper housing member. The two housing members12, 212, 14, 214 when joined, offer the advantages of a single housing11, 211 in that the tool assembly 10, 210 is more compact than a tie rodstructure and the housing 11, 211 can be cylindrical, allowing use ofthe tool assembly 10, 210 in narrow, confined locations such as betweenslabs, where the wider tie rod body would interfere with the slabs.

Although embodiments have been described with reference to a hammerassembly in which the drive means and return means are hydraulicpressure sources, the disclosed embodiments are applicable to any toolassembly having a piston movable within a chamber by suitable drivestructure and return structure. The disclosed embodiments encompasspneumatic tools and other impact tools.

While the disclosed embodiments have been illustrated and described indetail in the drawings and foregoing description, such illustration anddescription is to be considered as exemplary and not restrictive incharacter, it being understood that exemplary embodiments have beenshown and described and that all changes and modifications that comewithin the spirit of the disclosure are desired to be protected.

1. A method comprising: attaching an upper housing member to a lowerhousing member to form a housing, the housing including a chamberextending between the upper housing member and the lower housing member,the upper housing member having a first pressure source of pressurizedhydraulic fluid or pressurized air; positioning a piston inside thechamber for reciprocal movement therein, the piston including at least afirst surface which communicates with the first pressure source to movethe piston in a first direction; and positioning a reciprocating worktool at least partially inside of the chamber, an upper part of the worktool being struck by the piston as the piston moves in the firstdirection inside the chamber to transfer force from the piston to a tipof the work tool, the tip being formed opposite the upper part andresiding outside of the chamber, the upper part of the work tool beinghoused inside the lower housing member, wherein attaching the upperhousing member to the lower housing member further comprises engaging asocket portion formed on one of the upper housing member or the lowerhousing member with a plug portion formed on the other of the upperhousing member or the lower housing member, by inserting a plurality ofoutwardly projecting lugs formed on the plug portion into the socket andbetween a plurality of inwardly projecting projections formed on thesocket portion, then rotating the lugs relative to the projections toengage the lugs beneath the projections and prevent removal of the plugportion from the socket portion.
 2. A method according to claim 1further comprising: applying a prestressing force to urge the lowerhousing member in the first direction away from the upper housingmember.
 3. A method according to claim 2 further comprising: applyingthe prestressing force by turning a plurality of jacking screws evenlyspaced around the housing.
 4. A method according to claim 2 furthercomprising: applying the prestressing force by threading a plurality ofjacking screws into threaded bores formed on the upper housing member tobear against an upwardly facing mating surface on the lower housingmember.
 5. A method according to claim 2 further comprising: applyingthe prestressing force by urging a compressively loaded annularcompression sleeve positioned in the chamber against a bearing surfaceformed on the lower housing member.
 6. A method according to claim 5further comprising positioning the annular compression sleeve in thechamber between the piston and the upper housing member. 7.-9.(canceled)
 10. A hammer comprising: a housing including an upper housingmember and a lower housing member, the housing defining a chamber, theupper housing member having a first pressure source of pressurizedhydraulic fluid or pressurized air; a piston reciprocating inside thechamber, the piston including at least a first surface whichcommunicates with the first pressure source to move the piston in afirst direction; a reciprocating work tool residing at least partiallyinside of the chamber, an upper part of the work tool being struck bythe piston as the piston moves in the first direction inside the chamberto transfer force from the piston to a tip of the work tool, the tipbeing formed opposite the upper part and residing outside of thechamber, the upper part of the work tool being housed inside the lowerhousing member; a socket portion formed on one of the upper housingmember or the lower housing member, a plug portion formed on the otherof the upper housing member or the lower housing member, the plugportion positioned in the socket portion, a plurality of outwardlyprojecting lugs formed on the plug portion and a plurality of inwardlyprojecting projections formed on the socket portion, the lugs beingpassable between the projections when the upper housing member isattached to the lower housing portion by inserting the plug portion intothe socket portion, then rotatable relative to the projections to engagethe lugs beneath the projections and prevent removal of the plug portionfrom the socket portion; and a prestressing structure configured to urgesurfaces on the lugs and the projections into compressive engagementwith one another, the prestressing structure including a compressivelyloaded compression sleeve positioned in the chamber and urging the lowerhousing member away from the upper housing member by bearing against abearing surface formed on the lower housing member. 11.-13. (canceled)14. A hammer comprising: a housing including an upper housing member anda lower housing member, the housing defining a chamber, the upperhousing member having a first pressure source of pressurized hydraulicfluid or pressurized air; a piston reciprocating inside the chamber, thepiston including at least a first surface which communicates with thefirst pressure source to move the piston in a first direction: areciprocating work tool residing at least partially inside of thechamber, an upper part of the work tool being struck by the piston asthe piston moves in the first direction inside the chamber to transferforce from the piston to a tip of the work tool, the tip being formedopposite the upper part and residing outside of the chamber, the upperpart of the work tool being housed inside the lower housing member; atool retention member removably attached to the lower housing member,wherein when the tool retention member is attached to the lower housingmember the work tool cannot be removed from the chamber and when thetool retention member is removed from the lower housing member, the worktool may be removed from the chamber for replacement; engaging structureassociated with the upper housing member and the lower housing memberand configured to permit the engagement and disengagement of the upperhousing member with the lower housing member by relative rotation of theupper housing member and the lower housing member, the engagingstructure including mutually engaging mating surfaces on the upperhousing member and lower housing member; a prestressing structureconfigured to urge the lower housing member in the first direction awayfrom the upper housing member, the prestressing structure including acompressively loaded compression sleeve positioned in the chamber andurging the lower housing member away from the upper housing member bybearing against a bearing surface formed on the lower housing member.15. A method according to claim 1 further comprising: removablyattaching a tool retention member to the lower housing member, whereinwhen the tool retention member is attached to the lower housing memberthe tool retention member blocks removal of the work tool from thechamber and detachment of the work tool from the lower housing member,and when the tool retention member is removed from the lower housingmember, the work tool may be removed from the chamber and detached fromthe lower housing member for replacement.
 16. A hammer according toclaim 10, further comprising: a tool retention member removably attachedto the lower housing member, wherein when the tool retention member isattached to the lower housing member the work tool cannot be removedfrom the chamber and when the tool retention member is removed from thelower housing member, the work tool may be removed from the chamber forreplacement.
 17. A hammer according to claim 14, further comprising: atool retention member removably attached to the lower housing member,wherein when the tool retention member is attached to the lower housingmember the work tool cannot be removed from the chamber and when thetool retention member is removed from the lower housing member, the worktool may be removed from the chamber for replacement.
 18. A hammercomprising: a housing including an upper housing member and a lowerhousing member, the housing defining a chamber, the upper housing memberhaving a first pressure source of pressurized hydraulic fluid orpressurized air; a piston reciprocating inside the chamber, the pistonincluding at least a first surface which communicates with the firstpressure source to move the piston in a first direction; a reciprocatingwork tool residing at least partially inside of the chamber, an upperpart of the work tool being struck by the piston as the piston moves inthe first direction inside the chamber to transfer force from the pistonto a tip of the work tool, the tip being formed opposite the upper partand residing outside of the chamber, the upper part of the work toolbeing housed inside the lower housing member; an engaging structureassociated with the upper housing member and the lower housing memberand configured to permit the engagement and disengagement of the upperhousing member with the lower housing member by relative rotation of theupper housing member and the lower housing member, the engagingstructure including mutually engaging mating surfaces on the upperhousing member and lower housing member; and a prestressing structureconfigured to urge the lower housing member in the first direction awayfrom the upper housing member, the prestressing structure including atleast one compression member positioned between the upper housing memberand the lower housing member.
 19. The hammer according to claim 18,wherein the compression member comprises a compressively loadedcompression sleeve positioned in the chamber and urging the lowerhousing member away from the upper housing member by bearing against abearing surface formed on the lower housing member.
 20. A hammercomprising: a housing including an upper housing member and a lowerhousing member, the housing defining a chamber, the upper housing memberhaving a first pressure source of pressurized hydraulic fluid orpressurized air; a piston reciprocating inside the chamber, the pistonincluding at least a first surface which communicates with the firstpressure source to move the piston in a first direction; a reciprocatingwork tool residing at least partially inside of the chamber, an upperpart of the work tool being struck by the piston as the piston moves inthe first direction inside the chamber to transfer force from the pistonto a tip of the work tool, the tip being formed opposite the upper partand residing outside of the chamber, the upper part of the work toolbeing housed inside the lower housing member; an engaging structureassociated with the upper housing member and the lower housing memberand configured to permit the engagement and disengagement of the upperhousing member with the lower housing member by relative rotation of theupper housing member and the lower housing member, the engagingstructure including mutually engaging mating surfaces on the upperhousing member and lower housing member; and a prestressing structureconfigured to urge the lower housing member in the first direction awayfrom the upper housing member, the prestressing structure including atleast one tension screw.
 21. A hammer according to claim 20, wherein theprestressing structure further includes a compression sleeve urged bytension applied by the at least one tension screw into compressivecontact with a bearing surface on the lower housing member.
 22. A hammeraccording to claim 21, wherein the compression sleeve includes at leastone sleeve member having a resilient deflector flange.